Adiponitrile process



. N i O I FL 1- nJ FL C .L E E MANUEL M. VBAIZER CHARLES R. CAMPBELL ROBERT H. FARISS ROBERT JOHNSON lNVENTOR- United States Patent 01 sce- 3,193,480 Patented July 6, 1965 3,193,480 ADIPONITRILEv PROCESS Manuel M. Baizer. St. Louis, Mo., Charles R. Campbell,

Pensacola, F la. Robert H. Fariss, St. Louis, Mo., and

Robert Johnson, Pensacola. Fla, assignors to Monsanto Company, acorporntion of Delaware Filed Feb. 1, 1963, Ser. No. 255,586 10 Claims. (Cl. 204-73) The present invention relates to a process of electrolytically hydrodimerizing acrylonitrile to acliponitrile in a salt catholyte of a divided cell employing acid anolyte.

General conditions which are effective for electrolyzing acryl'onitrile to adiponitrile are described in co pending applications of one of us, Ser. No. 228,740, filed October 5-, 1962, and now abandoned. such application being a continuation-in-part of Ser. No. 145,482, filed October 16, 1961, and now abandoned; and of Ser. No.

75,130, filed December 12, 1960, and now forfeited, the

aforesaid applications having been abandoned or forfeited in favor of continuation-impart application SerfNo. 333,647, filed December 26, 1963. The present inven-' tion is concerned with the employment of acid anolyte in such a process, the conditions of such employment and the advantages resulting therefrom.

The figure of the drawing is an illustration of an acrylonitrile electrolysis system in which the anolyte and catholyte are separately circulated through the electrolysis cell which is illustrated in cross-section.

There are certain advantages in employing a divided cell in the electrolytic hydrodimerization of acrylonitrile. Forone thing; it is desirable to keep the acrylonitrile away from the anode Where it would be subject to oxidation. Similarly, many of the most suitable salt electrolytes for the hydrodimerization of acrylonitrile, for example quaternary ammonium aromatic sulfonates, are subject to decomposition at the anode. Such salts are particularly suitable as they have very negative cathodedischarge potentials and are therefore unlikely to interfere with the hydrodimerization of acrylonitrile at the cathode which occurs at about l.75 to about -1.9 volts (vs. a saturated calomel electrode). However, the presence of such salts or the anodic oxidation products of such salts and acrylonitrile in the anolyte can cause severe anode corrosion. A salt solution is used for the electrolysis because of the. tendency of acrylonitrile to polymerize when electrolyzed under acid conditions. It is generally desirable to have the catholyte pH in the range of about 7 to about 9.5 or 10.

The term consisting essentially of" as employed herein with respect to the solutions elcctrolyzed is intended could successfully be conducted for some period of time without provisions to counteract the alkalinity, it is apparent that eventually the build-up of hydroxyl ions in the catholylc would be such as to cause undesirable side reactions to predominate. Control of alkalinity becomes particularly necessary if the electrolytic hydrodimerization is conducted in a continuous manner with continuous or intermittent addition of nitrile and removal of product, while the electrolyte itself stays in the cell or is recycled to the cell. When a divided cell utilizes a cationic diaphragm and acid is employed as anolyte, the alkalinity of the catholyte depends upon the rate at which ions or molecules move across the diaphragm relative to the rate phragm as a result of concentration differences. 2h

ln accordance with the present invention it has been found that inthe electrolysis of acryl'onitrile control of I catholyte alkalinity can very suitably be effected by employing a mineral acid anolyte in conjunction with a cation cxchange membrane separating the anolyte from the catholyte. I I

In the past various electrolysis reactions for reducing or otherwise altering organic compounds have been known. In general, however, such reactions have had the disadvantage of being of small scale and low velocity and requiring careful control of many conditions. Quite often such reactions could not be scaled-up by using high current densities to give practical production rates and therefore remained laboratory curiosities. In contrust, the present process operates very effectively at current densities of greater than 10 amperes/square deciineter of cathode surface, and the most suitable densities may be in the range of to to or 50 amperes/square decimeter and higher, even up to 100 or more amperes/ to leave the solutions open to addition of other components which do not change the basic nature of the solutions with respect to the electrolytic hydrodimerization being conducted therein.

There are many advantages in the use of an acid anolyte in the hydrodimcrization of acrylonitrile. Relatively speaking. an aqueous solution containing a quaternary ammonium salt and acrylonitrile tends to have a high electrical resistance. The overall resistance of the cell can be considerably lessened, therefore, if a mineral acid rather than a quaternary ammonium salt solution is utilized as anolyte. Mineral acids. e.g.. sulfuric acid. have the additional advantage of being inexpensive and readily available. Phosphoric acid and other mineral acids can be employed, as can organic sulfonic acids, e.g., benzene or toluene sulfonic acids. Sulfuric acid is definitely preferred as its use minimizes corrosion problems and the oxygen produced at the anode in such acid does not cause any special difiiculties. During electrothe catholyte. While hydrodimerization of acrylonitrile square decimeter, and it is further possible to use cells having a large eflective electrode area, whether in a single set of electrodes or in a series of electrodes. Thus in commercial practice it is probable that individual cells will draw at least 20 to 30 ampe'res, most likely more than amperes, and cells drawing more than 1000 amperes are contemplated. For reasons of economics and to make practical use of such current densities without ne cessitating prohibitively high cell voltages, it is essential to have fairly low resistance in the cell as obtainable by utilizing fairly high concentrations of the electrolyte salt and a relatively narrow gap between the .electrodes, e.g., no more than one-half inch. and preferably of the order of one-fourth inch or smaller. Applied voltages of 5 to 20 volts for current densities of 15 to 40 amperes/ dm. are suitable, and it is preferable, in this range as well as at higher densities that the applied voltage have a numerical value no greater than one-half of the numerical value of the current density (in amperes/dmF). Various power sources are suitable for use in the present invention. particularly any efficient sources of direct current, and, if desired, various known means of varying the applied potential to regulate the current density and the cathode potential can be employed, for example the means described in Metcalf et al., US. Patent No. 2,835,631, issued May 20, 1958, the disclosure of which is incorporated herein by reference. If desired altcrnat ing current can be superimposed on the direct current applied to the cell.

In carrying out the present invention, the acid employed as anolyte will have a normality such that enough hydrogen ions migrate to the catholyte to counteract the hydroxyl ions generated at the chosen current density.

Example Acrylonitrilewas hyd'rodimerized in a divided cell with acid anolyte in a number of runs under comparable conditions except that the current density was varied and the anolytenormality was correspondingly adjusted to control catholyte pH, with results as follows:

Current Anolytn Catltolyte Run Density. Normality pll amps/din? I i I i 1 (1.5; i ll. ll 20 i it. til a. 7 J5 t'tstl 5.0 31! 1 ll. 95 i 7 40 i 1.08 s7 The foregoing runs were made with sulfuric acid "as anolyte and employing a cationic membrane as the cell divider, the particular membrane being a cationic mem- All of the runs were successfully sulfonate in water, and the acrylonitrile was provided in an amount to constitute 33% by weight of the catholyte. While there will be some variation with the ion exchange membranes employed and other electrolysis conditions, it will generally be desirable to'have the acid anolyte' normality have a value in the range of about 0.02 to 0.06 times the current density (in ampe'rcs/dmF), for example, about 0.03 times the current density, in order to maintain the catholyte pH relatively constant. For example, normalities of about 0.4 to 1.2 are suitable for current densities of to 40 ampe'rcs/dm. of cathode area. In general the acid employed as 'anolyte will be fairly dilute, particularly when a strong mineral acid is employed. e.g., from 0.05 to 5 or 10% by weight acid based on the weight of acid and water, and seldom more than by weight on the same basis. Sulfuric acid is very suitable for use in the present invention, and such concentrations of sulfurie acid can suitably be employed, and'are practical and useful from the viewpoint of conductivity and low anodic corrosion, as well as pH control.- Other strong acids, e.g.. those having relatively high ionization constants,

' such as greater than lO- at C., cart suitably be embrane comprised of. a sulfonated styrcne-divinyl benzene polymer supported upon a glass fiber base. The lnear flow rate of the catholyte along the cathode surface was approximately 1.5 ft./sec.

The electrolyses were conducted in the systemillustrated in the figure of the drawing.

The electrolysis takes place in electrolysis cell 1 in which a cathode plate 2 and a diaphragm 3 with the other Walls of the completely enclosed cell chamber form a cathode compartment and the said diaphragm 3 and an ode plate 4 form a separate anode compartment. cathode 2 is connected to the negative terminal of a source of direct current. while the anode 4 is connected to the positive source'of, such current, and the cathode and anode are clamped in juxtaposition but separated by insulating gasketing material. ,In operation, acrylonitrile is fed through pump 5 and the sal't'solution through pump 6 to the catholyte recirculating pump 7 and then through an aperture in the cathode plate 2, through the cathode cornpartment and out another aperture in the cathode plate to the catholyte reservoir 8. The catholyte reservoir is provided with means (not illustrated) such as a gravity overflow system to discharge part of the catholyte, and means are also available to heat the catholyte in the reservoir if necessary. A pH meter 9 is hooked into the catholyte recirculationline 10, and a water-jacketed heat exchanger 11 is hooked across the recirculation pump to permit cooling of.the catholyte if necessary. The anolyte is made up by feeding water and acid to the anolyte reservoir 12 from which it is fed through a water-jacketed heat exchanger 13 to the anolyte recirculating pump 14. thence through an aperture in the anode plate 4 and through the anode chamber to return to the anolyte reservoir 12. The anolyte reservoir is equipped with a water-jacketed condenser 15 to permit oxygen or other gases to escape from the anolyte. There is a differential manometer 16 connected across the catholyte and anolyte inlets of cell 1 to permit equalization of pressures on the two sides of diaphragm 3. The cathode .was lead and the anode,platinum on titanium; other electrode materials are suitable, forexample, mercury or various lead alloys as cathode, and lead or lead alloys as anode. It will be understood that the electrodes are in actual contact, i.e., solid-liquid contact, with the catholyte and anolyte and are not separated therefrom by an air gap or otherwise.

The catholyte employed in the runs was a by weight concentration of tetrarnethylamrnonium toluene The Fit)

ployed in such concentrations.

Numerous electrolyte salts are already known and suitable electrolyte salts can be selected in the light of the present disclosure for use in the catholyte in the present invention. In general it will be desirable to avoid overly acidic or overly basicsalts in view of the pH considerations discussed herein, and ordinarily salts of strong bases will be employed, particularly salts of strong bases and strong acids.

Among the salts which can be employed the amine and quaternary ammonium salts are generally suitable, especially those of sult'nnic and alkyl sulfuric acid. Such salts can he the saturated aliphatic amine salts or heterocylic amine salts. c.g.. the mono-, dior trialkylamine salts, or the mono-. dior trialkanolamine salts. or the piperidine, pyrrolidine or morpholine salts, e.g., the cthylamine, dimcthyamine or triisopropylamine salts of various acids. especially various sulfonic' acids. Especially preferred are aliphatic and hctcrocyclic quaternary ammonium salts. i.e.. the tetraalkylammonium or the tetraalkanolammonium salts or mixed alkyl alkanol ammonium salts such as the alkyltriakanolammoni um, the dialkyldialltanolammonium, the alkanotrialkylammonium or the N-heterocyclic N-alkyl' ammonium salts of sulfonic or other suitable acids. The saturated aliphatic or heterocyclic quaternary ammonium cations in general have suitably high cathode discharge potentials for use in the present invention and readily form salts having suitably high. water solubility with anions suitable for use in the electrolytes employed in the present invention. The saturated, aliphatic or heterocyclic quaternary ammonium salts of such acids are therefore in general well adapted to dissolving high amounts of olcfinic compounds in their aqueous solutions, i.e., hydrotropic salts, and to effecting reductive couplings of such olefinic compounds. It is understood, of course, that it is undesirable that the ammonium groups contain any reactive groups which might interfere to some extent with the reductive coupling reaction. In this connection it should be noted that aromatic unsaturation as such does not interfere as benzyl substituted ammonium cations can be'employed (as also can aryl sulfonate anions).

Among the anions useful in the electrolytes, the aryl and alkaryl sulfonic acids are especially suitable, for example. salts of the following acids: lw'cnzenesulfonic acid, o-. mor p-toluencsulfonic acid, o-. m or p-cthylbenzenesulfonic acid. o-, mor p-cumenesulfonie acid, o-, mor p-tcrt-amylbenzenesultonic acid, o-, mor p-hexylbenzenesulfonic acid, o-xylenel-sulfonic acid, p-xylene-Z- sulfonic acid, m-xylene-4 or 5 sulfonic acid, mesitylene-Z- sulfonic acid, durcnc-Z-sulfonic acid. pentamethylbenzenesulfonie acid, o-dipropylbcnzenet-sultonic acid, alphaor beta-naphthalenesulfonie acid, o-, mor p-biphenylsulfonic acid, and alpha-methyl-beta-naphthalenesulfonic acid. Alkali metal salts are useful with certain limitations, and the alkali metal salts of such sulfonic acids can be employed, i.e., the sodium, potassium, lithium, cesium, or rubidium salts such as sodium benzenesulfonate, potassium p-toluenesulfonate, lithium o biphenylsulfonate, rubidium beta naphthalenesulfonate, cesium p-ethylbenezenesulfonate, sodium o-xylene-3-su]fonate, or potassium pentamethylhenzenesulfonate. The salts of such sulfonic acids may also be the saturated, aliphatic amine or heterocyclic amine salts, e.g., the monodior trialkylamine salts, or the mono-, di or trialkanolamine salts, or the piperidine, pyrrolidine, or morpholine salts, e.g., the ethylamine, dimethylamine or triisopropylamine salt of benzenesulfonic acid or of 0-, por m-toluenesulfonic acid; the isopropanolamine, dibutanolamine or triethanolamine salt of o-, por m-toluenesulfonic acid or of o-, p-, or m-biphenylsulfonic acid; the piperidine salt of alphaor beta-naphthalenesulfonic acid or of the cumenesulfonic acids, the pyrrolidine salt of o-, m-, or pamylbenzenesulfonate; the morpholine salt of benzenesulfonic acid, of o-, mor p-toluenesulfonic acid, or of alphaor beta-naphthalenesulfonic acid, etc. In general, the sulfonates of any of the ammonium cations disclosed generically or specifically herein can be employed in the present invention. The aliphatic sulfonates are prepared by reaction of the correspondingly substituted ammonium hydroxide with the sulfonic acid or with an acyl halide thereof. For example, by reaction of a sulfonic acid such as p-toluenesulfonic acid with a tetraalkylammonium Q hydroxide such as tetraethylammonium hydroxide there is obtained tetraethylammonium p -toluenesulfonate, use i of which in the presently provided process has been found to give very good results. Other presently useful quaternary ammonium 'sulfonates are, e.g., tetraethylammoni- T urn 0- or m-toluenesulfonate or benzenesulfonate; tetraethyl-ammonium o-, mor p-cumenesulfonate or o-, m-, or I p-ethylbenzenesulfonate, tetraethylammonium benzenesulfonate, or o-, mor p-toluenesulfonate; N, N-di-methylpiperidinium o-, mor p-toluenesulfonate or o-, m or pbipyhenylsulfonate; tetrabutylammonium alphaor betanaphthalenesulfonate or o-, mor p-toluenesulfonate; tetrapropylammonium o-, mor alpha-, ethylbeta-naphthalenesulfonate; tetraethanol ammonium o-, mor tulenesulfonate; tetrabutanolammonium benzenesulfonate or p-xylene-3-sulfonate; tetrapentylammonium o-, mor p-toluenesulfonate or o-, mor p-hexylbenzenesulfonate, tetrapentanolammonium p-cumene-3-sulfonate or benzenesulfonate; methyltriethylammonium o-, mor p-toluenesulfonate or mesitylene-Z-sulfonate; trimethylethylammonium o-xylene-4-sulfonate or o-, mor p-toluenesulfonate; triethylpentylammonium alphaor beta-naphthalenesulfonate or o-, mor p-butylbenzenesulfonate, trimethylethanolammonium benzenesulfonate or o-, mor p-toluenesulfonate; N,N-diethylpiperidinium or N-methylpyrrolidinium o-, mor p-hexylbenzenesulfonate or o-, mor p-toluenesulfonate, N,N-di-isopropyl or N,N-dibutylmorpholinium, o-, mor p-toluenesulfonate or o-, mor p-biphenylsulfonate, etc.

The tetraalkylammonium salts of the aryl or alkarylsulfonic acids are generally preferred for use as the salt constituents of the electrolysis solution because the electrolyses in the tetraalkylammonium sulfonates are exclusively electrochemical processes.

Among the ammonium and amine sulfonates useful as electrolytes in the present invention are the alkyl, aralkyl, and heterocyclic amine and ammonium sulfonates, in which ordinarily the individual substituents on the nitrogen atom contain no more than atoms, and usually the amine or ammonium radical contains from 3 to carbon atoms. It will be understood, of course, that diand poly-amines and diand poly-ammonium radicals are operable and included by the terms amine and ammonium. The sulfonate radical can be from aryl, alkyl,

or p-amylbenzenesulfonate p-cumenesulfonate or o-, mor palkaryl or aralkyl sulfonicacids of various molecular weights up to for example 20 carbon atoms, preferably about 6 to 20 carbon atoms, and can include one, two or more sulfonate groups.

Various other cations are suitable for use in the present invention, e.g., tetraalkyl-phosphonium and trialkylsult'onium cations, particularly from sulfonate salts formed from sulfonic acids as described above. Similar- 1y, other anions can be selected from alkylsulonates and anions of other acids. Further guides to the choice and use of the electrolyte salt are found in the aforementioned Ser. No. 228,740.

In effecting the present invention it is preferred to employ a permselective membrane as the cell divider, particularly a membrane which is permselective as to particles carrying a positive electric charge and which therefore permits the passage of cations, while preventing the passage of undesirable amounts of anions. Continuous, non-porous permselective cationic membranes comprising insoluble cation-exchange resin are suitable.

The cation-exchange resins which are preferred for use in such membranes are those of the sulfonic and carboxylic types, many of which are available commercially, such as sulfonated phenolaldehyde resin products, sulfonated cross-linked polymers of styrene, and carboxylic resins such as typified by those included in U.S. Patents Nos. 2,340,110 and 2,340,111, or any of the resins disclosed as suitable for such membranes in Juda et al., Reissue Patent 24,865. For example, a cationic membrane comprised of a sulfonated styrene-divinyl benzene polymer supported upon a glass fiber or fabric material can be employed. In general the cation permselective membranes employed comprise a solid polymeric matrix with at least 1 milliequivalent per dry gram of membrane of dissociable ionic groups, e.g., acid groups such as carboxylate group's, fixed into said matrix, said membrane being reinforced by an embedded non-corrodible material. A suitable polymer of the indicated type is the sulfonated styrene-divinyl benzene copolymer described under the name Dowex 50 in the Journal of the American Chemical Society, volume 69, page 2830, (1947). Various other types of exchange materials can be empl0yed,.for example, porous film cast from a homogeneous mixture of 70-90% by weight of vinyl chlorideacrylonitrile copolymer and 30-10% by weight of a watersoluble poly (vinyl benzyl trialkylammonium salt); membranes of sulfonated vinyl chloride polymers and copolymers; polyacrylic membranes and membranes of other olefinic acid polymers, and other addition polymers of carboxylic acids, carboxylic anhydrides, carboxylic esters, carboxylic amides, carboxylic chlorides, carboxylic nitriles and other compounds which contain or can be hydrolyzed to carboxyl groups; membranes of condensation polymers of methylol forming sulfonated alkyl aryl ethers with aldehydes, or of various sulfonated phenol-aldehyde resins. homogeneous, uniform films of the ion exchange material, or can be uniform and intimate dispersions of such ion exchange material in afilm or matrix of, for example, a polymer of a perhaloethylene such as trifluorochloro ethylene or tetrafiuoroethylene; or the membranes can be particles of ion exchange material formed together by the same or a different type of resin or by water-insoluble jelly-like material such as petrolatum, amorphous wax, hydrocarbon jels, etc. It is preferred that the membrane be substantially non-porous in order that it may be sufiiciently permselective and will not permit too rapid movement of ions. It will be understood that the membranes must be reasonably resistant to attack by both anolyte and catholyte solutions under conditions of operation and also that good mechanical strength is necessary for the embodiment in which there is highly turbulent liquid flow over the membrane surface.

What is claimed is:

l. The method of electrolyzing acrylonitrile to obtain The membranes employed can be substantially and recover adiponitrilc which comprises passing electric current through a catholyte in contact with a cathode, the

catholyte consisting essentially of water, acrylonitrile and an electrolyte salt which has a discharge potential more negative than that of acrylonitrile and being separated by a diaphragm from the anolyte which is an aqueous acid solution having an acid concentration of 0.05 to 20% by weight, and of strength matched to the current density to provide sumcient hydrogen ion to neutralize hydroxyl ionsgen erated at the cathode, and recovering adiponitrile from the catholyte.

2. The method of claim 1 in which the anolyte is an aqueous solution of an inorganic acid and the anolyte is separated from the catholyte by an ion exchange membrane.

3. The method of claim 1 in which the normality of the acid is in the range of 0.4 to 1.2 and the current density is in the range of 15 to 40 amperes per square decimeter.

4. The method of electrolyzing acrylonitrile to obtain and recover adiponitril'e which comprises passing electric current througha catholyte in contact with a cathode, the catholyte being an aqueous electrolyte salt solution containing acrylonitrile, the electrolyte salt having a discharge potential more negative than that of acrylonitrile, and the catholyte'being separated by. an ion exchange membrane from the anolyte which is an aqueous'acid solution of acidity such that thepH of the catholyte is maintained in the range of 7 to 9.5, and recovering adiponitrile from the catholyte.

, The method oficlaim 1 in which the anolyte is an aqueous solution of an inorganic acid which is separated from the catholyte by a perm-selective cationic menibrane and the electrolysis is conducted on a continuous basis involving addition of acrylonit'rile to the catholyte and removal of adiponitrile from the catholyte during the elcctrolysis'without the necessity of adding materials to the catholyte to adjust the hydrogen ion concentration thereof.

6. The method of claim 5 in which the 5 aqueous sulfuric acid solution.

7. The method of claim 6 in which the salt isa hydrotropic quaternary ammonium salt.

8. The method of claim 1 in which the acid is sulfuric acid. A

9. The method ofclaim 4 in which the cathode is composed of lead.

10. The method of electrolyzing acrylonitrile which comprises passing electric current through an aqueous quaternary ammonium sulfonate solution containing acrylonitrile in contact with a lead metal cathode, the said anolyte is an solution being separated by an ion exchange membrane References Cited by the Examinerv UNITED STATES PATENTS 2,726,204 12/55 Park etal. 204- 72 2,921,005 '1/60 BOdatnet' 204-72 FOREIGN PATENTS 566,274 11/58 Canada.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, WINSTON A.

. Examiners.

DOUGLAS, 7

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,193,480 July 6, 1965 Manuel M. Baizer et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, lines 17 and 18, for "increated" read increased column 4, line 39, for "dimethyamine" read dimethylamine column 5, line 38, for "tetraethylammonium" read tetramethylammonium line 46, for "tulenesulfonate" read toluenesulfonate line 49, for "cumene-" read cymenecolumn 6, line 9, for "alkylsulonates" read alkylsulfates Signed and sealed this 18th day of October 1966.

EAL) ttest:

RNEST W. SWIDER EDWARD J. BRENNER Iesting Officer Commissioner of Patents 

1. THE METHOD OF ELECTROLYZING ACRYLONITRILE TO OBTAIN AND RECOVER ADIPONITRILE WHICH COMPRISES PASSING ELECTRIC CURRENT THROUGH A CATHOLYTE IN CONTACT WITH A CATHODE, THE CATHOLYTE CONSISTING ESSENTIALLY OF WATER, ACRYLONITRILE AND AND ELECTROLYTE SALT WHICH HAS A DISCHARGE POTENTIAL MORE NEGATIVE THAN THAT OF ACRYLONITRILE AND BEING SEPARATED BY A DIAPHRAGM FROM THE ANOLYTE WHICH IS AN AQUEOUS ACID SOLUTION HAVING AN ACID CONCENTRATION OF 0.05 TO 20% BY WEIGHT, AND OF STRENGTH MATCHED TO THE CURRENT DENSITY TO PROVIDE SUFFICIENT HYDROGEN ION TO NEUTRALIZE HYDROXYL IONS GENERATED AT THE CATHODE, AND RECOVERING ADIPONITRILE FROM THE CATHOLYTE. 